Elevated-Temperature R-Curve Behavior of a Polycrystalline Alumina

The fracture behavior of a polycrystalline alumina was examined at temperatures ranging from ambient through 1400°C, using three‐point bend bar test specimens. R‐curves were determined at all temperatures studied, and when accompanied by renotching procedures, a wake removal technique, conclusive evidence was provided to support the existence of a following wake region in this monolithic ceramic material. The crack closure stresses identified in this region are responsible for all toughening with crack extension observed in this study. Room‐temperature “KIC” fracture toughness values of 4.5 MPa · m1/2 for the chevron‐notch specimen and 3.9 MPa · m1/2 for the straight‐notch configuration were obtained. The critical stress intensity factor of the renotched chevron‐notch specimen compared very closely with that of the straight‐notch specimen. These findings further confirm the toughening role of the microstructural features found in the following wake region. This paper considers, in detail, these observations in terms of the microstructure and its role in the toughening mechanism.